Multistage process for the polymerization of olefins

ABSTRACT

Process for transferring polyolefin particles from a first gas-phase polymerization reactor to a second gas-phase polymerization reactor in a multistage polymerization of olefins carried out in at least two serially connected gas-phase polymerization reactors, 
     wherein the first gas-phase reactor is a fluidized-bed reactor comprising a gas distribution grid and a settling pipe, which is integrated with its upper opening into the distribution grid and contains a bed of polyolefin particles which moves from top to bottom of the settling pipe,
 
the process comprising the steps of
 
introducing a fluid into the settling pipe in an amount that an upward stream of the fluid is induced in the bed of polyolefin particles above the fluid introduction point;
 
withdrawing polyolefin particles from the lower end of the settling pipe; and
 
transferring the withdrawn polyolefin particles into the second gas-phase polymerization reactor,
 
process for polymerizing olefins comprising such a process for transferring polyolefin particles, reactor suitable as first gas-phase polymerization reactor in the process for polymerizing olefins and process for discharging polyolefin particles from a fluidized-bed reactor.

This application is the U.S. National Phase of PCT InternationalApplication PCT/EP2012/074324, filed Dec. 4, 2012, claiming benefit ofpriority to European Patent Application No. 11192070.8, filed Dec. 6,2011, and claiming benefit of priority under 35 U.S.C. §119(e) to U.S.Provisional Application No. 61/569,524, filed Dec. 12, 2011, thecontents of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to a process for transferring polyolefinparticles from a first gas-phase polymerization reactor to a secondgas-phase polymerization reactor in a multistage polymerization ofolefins carried out in at least two serially connected gas-phasepolymerization reactors. It further relates to a process forpolymerizing olefins comprising such a process for transferringpolyolefin particles. Moreover, the invention relates to a reactorsuitable as first gas-phase polymerization reactor in the process forpolymerizing olefins and to a process for discharging polyolefinparticles from a fluidized-bed reactor.

BACKGROUND OF THE INVENTION

The polymerization of olefins in two or more serially connectedgas-phase reactors allows to produce olefin polymers with improvedproperties and/or to simplify the existing production processes. This ismade possible by choosing polymerization conditions in the secondreactor or subsequent reactors different from the reaction conditionsexisting in the first polymerization reactor. Typically, olefin polymersgrow on particles including a catalyst component, which continues toexert a catalytic activity even when the polymer particles aretransferred to a successive gas-phase reactor. The polymer resultingfrom the first gas-phase reactor is transferred to the second gas-phasereactor, where polymerization is continued under different conditions.Therefore, different fractions of polymer can grow on the same particleby maintaining a different composition of the gas-phase mixture in eachreactor.

Examples of polymers that may be produced by a multistage gas-phaseprocess include bimodal or multimodal polymers obtained by maintaining adifferent concentration of a chain terminator, such as hydrogen, in eachreactor; and random or heterophasic copolymers obtained by polymerizingdifferent (co)monomers in each reactor. The term “heterophasiccopolymer” includes also in-reactor polymer blends.

The transfer of the polymer from one gas-phase reactor to another one isa critical step of a multistage polymerization process. A directdischarge of polymer from an upstream reactor to a downstream reactordoes not allow maintaining really different polymerization conditions inthe downstream reactor, due to the substantial amount of gases anddissolved hydrocarbons associated to the polymer transferred to thedownstream reactor.

A solution that has been proposed for a long time is degassing the solidpolymer discharged from the upstream reactor, then subjecting thepolymer to a compression stage and transferring it to the downstreampolymerization reactor. A process according to that solution isdisclosed in EP 192 427 A1, which describes a process in which thecompression stage is performed by means of the reaction gas mixture ofthe downstream reactor at a temperature lower by at least 20° C. thanthe temperature of the downstream reactor. WO 2008/058839 A2 discloses aprocess for the multistage polymerization of olefins which allowscontinuously discharging the polymer and the gas reaction mixture fromthe upstream reactor into a transfer device and continuously feedingpolymer from the transfer device to a downstream reactor by using atransfer device comprising a separation chamber, in which the gasreaction mixture is removed from the polymer, and at least a couple oflock hoppers, which work intermittently in parallel.

EP 050 013 A2 refers to a process for polymerizing an olefin in thegaseous phase in a multiplicity of steps in at least two independentpolymerization zones connected to each other by a transfer passage bywhich a gaseous stream containing the polymer obtained in a firstpolymerization zone is transferred into a second polymerization zone.The process is characterized in that an inert gas zone is provided inthe transfer passage and at least a part of the gas components of thegaseous stream containing the polymer is replaced by an inert gas.

These processes have the disadvantage that they comprise a pressurereduction step by which the reaction gas, which was removed with thepolymer from the first reactor, is separated from the polymer particles.However, for recycling the reaction gas to the upstream reactor it isneeded to compress this reaction gas again. This requires specificequipment and makes the process expensive and energy-intensive.

EP 1 040 868 A2 discloses a method of multistage gas phasepolymerization in which a polymerization of a feed gas mixture at leastcontaining ethylene, an alpha-olefin and hydrogen is carried out in anupstream arranged fluidized-bed reactor. The polymer powder taken upfrom the upstream arranged fluidized-bed reactor is treated with a gasto lower the content of alpha-olefin gas and hydrogen gas in thepolymers powder and then introduced into a downstream arranged reactor.

U.S. Pat. No. 7,465,772 B2 describes a method for continuouslypolymerizing olefin(s) in a plurality of serially-disposed gas-phasepolymerization reactors in which an upstream and a downstream reactorbeing adjacent to each other are connected via a gas exchange vesselcontaining a gas distributor plate. The polymer powder transferred fromthe upstream reactor is temporarily accumulated in the gas exchangechamber and the first gas which has been introduced from the upstreamreactor together with the polymer powder and which exists in the polymerpowder is exchanged at least partly with a second gas which is fed intothe gas exchange vessel. The polymer powder is then transferredintermittently from the gas exchange vessel to the downstream reactor.

US 2010/0029867 A1 discloses a gas-phase polymerization apparatuscomprising a gas-phase polymerization reactor and a gas separator whichis connected to the gas-phase polymerization reactor by a transfer tube.A mixture of a polymer powder and a gas is introduced into the gasseparator in which the gas contained in the mixture is replaced by areplacement gas. However, the disclosed set-up requires beside thereactor an additional pressurized vessel, the gas separator, and has therisk that polymer powder gets stuck in the transfer tube.

WO 2007/071527 A1 describes a method of discharging polymer particlesfrom a fluidized-bed reactor in which the polymer is continuouslyrecycled in an outside circulation loop from the gas distribution gridto the upper region of the fluidized-bed reactor and the polymer iswithdrawn from the circulation loop. WO 2008/074632 A1 discloses a gasdistribution grid of a fluidized-bed reactor having the inlet of thedischarge conduit been placed at the center of the distribution grid.The polymer particles discharged through this conduit are fed to thedegassing and extruding facilities.

Thus, it was the object of the present invention to find a simpleprocess for transferring polyolefin particles from a first gas-phasepolymerization reactor to a second gas-phase polymerization reactorwhich not only reliably allows to prevent the transfer of the reactiongas mixture of the first gas-phase reactor to the second gas-phasereactor, but which process also facilitates continuous polymerizationsin both the first and the second gas-phase polymerization reactor andwhich does not require a lot of machinery, i.e. can be implementedwithout high investment costs, and can be carried without the necessityof recompressing or recycling a larger amount of the reaction gas of thefirst gas-phase polymerization reaction, i.e. can be operated a lowoperational costs.

SUMMARY OF THE INVENTION

We found that this object is achieved by a process for transferringpolyolefin particles from a first gas-phase polymerization reactor to asecond gas-phase polymerization reactor in a multistage polymerizationof olefins carried out in at least two serially connected gas-phasepolymerization reactors,

wherein the first gas-phase reactor is a fluidized-bed reactorcomprising a gas distribution grid and a settling pipe, which isintegrated with its upper opening into the distribution grid andcontains a bed of polyolefin particles which moves from top to bottom ofthe settling pipe,

the process comprising the steps of

introducing a fluid into the settling pipe in an amount that an upwardstream of the fluid is induced in the bed of polyolefin particles abovethe fluid introduction point;

withdrawing polyolefin particles from the lower end of the settlingpipe; and

transferring the withdrawn polyolefin particles into the secondgas-phase polymerization reactor.

Furthermore, we have found a process for polymerizing olefins attemperatures of from 30° C. to 140° C. and pressures of from 1.0 MPa to10 MPa in the presence of a polymerization catalyst in a multistagepolymerization of olefins in at least two serially connected gas-phasepolymerization reactors comprising such a process for transferringpolyolefin particles. We also found a reactor for polymerizing olefinsin a fluidized bed of polyolefin particles comprising a gas distributiongrid, a velocity reduction zone and recycle gas line for transferringreaction gas from the top of the velocity reduction zone to a part ofthe reactor below the gas distribution grid, the recycle gas line beingequipped with a compressor and a heat exchanger, wherein the reactorfurther comprises a settling pipe which is integrated with its upperopening into the distribution grid and which is equipped with an inletfor introducing a fluid into the settling pipe and we found a processfor discharging polyolefin particles from such a fluidized-bed reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention can be betterunderstood via the following description and the accompanying drawingswhere FIGS. 1 and 2 show schematically set-ups for a multistagegas-phase polymerization of olefins.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for transferring polyolefinparticles from a first gas-phase polymerization reactor to a secondgas-phase polymerization reactor in a multistage polymerization ofolefins. Suitable olefins for such a polymerization are especially1-olefins, i.e. hydrocarbons having terminal double bonds, without beingrestricted thereto. Suitable olefins monomers can however also befunctionalized olefinically unsaturated compounds such as ester or amidederivatives of acrylic or methacrylic acid, for example acrylates,methacrylates, or acrylonitrile. Preference is given to nonpolarolefinic compounds, including aryl-substituted 1-olefins. Particularlypreferred 1-olefins are linear or branched C₂-C₁₂-1-alkenes, inparticular linear C₂-C₁₀-1-alkenes such as ethylene, propylene,1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-decene or branchedC₂-C₁₀-1-alkenes such as 4-methyl-1-pentene, conjugated andnonconjugated dienes such as 1,3-butadiene, 1,4-hexadiene or1,7-octadiene or vinylaromatic compounds such as styrene or substitutedstyrene. It is also possible to polymerize mixtures of various1-olefins. Suitable olefins also include ones in which the double bondis part of a cyclic structure which can have one or more ring systems.Examples are cyclopentene, norbornene, tetracyclododecene ormethylnorbornene or dienes such as 5-ethylidene-2-norbornene,norbornadiene or ethylnorbornadiene. It is also possible to polymerizemixtures of two or more olefins.

The process is in particular suitable in the multistagehomopolymerization or copolymerization of ethylene or propylene. Ascomonomers in ethylene polymerization, preference is given to using upto 40 wt.-% of C₃-C₈-1-alkenes, in particular 1-butene, 1-pentene,1-hexene and/or 1-octene. Preferred comonomers in propylenepolymerization are up to 40 wt.-% of ethylene and/or butene. Particularpreference is given to a process in which ethylene is copolymerized withup to 20 wt.-% of 1-hexene and/or 1-butene.

The transfer of polyolefin particles according to the present inventiontakes places from one gas-phase reactor to another gas-phase reactor ina multistage polymerization of olefins, where the first gas-phasereactor is a fluidized-bed reactor comprising a gas distribution gridand a settling pipe, which is integrated with its upper opening into thedistribution grid.

Fluidized-bed polymerization reactors are reactors in which thepolymerization takes place in a bed of polymer particles which ismaintained in a fluidized state by feeding in reaction gas at the lowerend of a reactor, usually below a gas distribution grid having thefunction of dispensing the gas flow, and taking off the gas again at itsupper end. The reaction gas is then returned to the lower end to thereactor via a recycle line equipped with a compressor and a heatexchanger. The circulated reaction gas is usually a mixture of theolefins to be polymerized, inert gases such as nitrogen and/or loweralkanes such as ethane, propane, butane, pentane or hexane andoptionally a molecular weight regulator such as hydrogen. The use ofnitrogen or propane as inert gas, if appropriate in combination withfurther lower alkanes, is preferred. The velocity of the reaction gashas to be sufficiently high firstly to fluidize the mixed bed of finelydivided polymer present in the tube serving as polymerization zone andsecondly to remove the heat of polymerization effectively. Thepolymerization can also be carried out in a condensing orsuper-condensing mode, in which part of the circulating reaction gas iscooled to below the dew point and returned to the reactor separately asliquid and gas-phase or together as two-phase mixture in order to makeadditional use of the enthalpy of vaporization for cooling the reactiongas.

In gas-phase fluidized-bed reactors, it is advisable to work atpressures of from 0.1 to 10 MPa, preferably from 0.5 to 8 MPa and inparticular from 1.0 to 3 MPa. In addition, the cooling capacity dependson the temperature at which the polymerization in the fluidized bed iscarried out. The process is advantageously carried out at temperaturesof from 30 to 140° C., particularly preferably from 65 to 125° C., withtemperatures in the upper part of this range being preferred forcopolymers of relatively high density and temperatures in the lower partof this range being preferred for copolymers of lower density.

The polymerization of olefins can be carried out using all customaryolefin polymerization catalysts. That means the polymerization can becarried out using Phillips catalysts based on chromium oxide, usingtitanium-based Ziegler- or Ziegler-Natta-catalysts, or using single-sitecatalysts. For the purposes of the present invention, single-sitecatalysts are catalysts based on chemically uniform transition metalcoordination compounds. Particularly suitable single-site catalysts arethose comprising bulky sigma- or pi-bonded organic ligands, e.g.catalysts based on mono-Cp complexes, catalysts based on bis-Cpcomplexes, which are commonly designated as metallocene catalysts, orcatalysts based on late transition metal complexes, in particulariron-bisimine complexes. Furthermore, it is also possible to usemixtures of two or more of these catalysts for the polymerization ofolefins. Such mixed catalysts are often designated as hybrid catalysts.The preparation and use of these catalysts for olefin polymerization aregenerally known.

Preferred catalysts are of the Ziegler type preferably comprising acompound of titanium or vanadium, a compound of magnesium and optionallya particulate inorganic oxide as support.

As titanium compounds, use is generally made of the halides or alkoxidesof trivalent or tetravalent titanium, with titanium alkoxy halogencompounds or mixtures of various titanium compounds also being possible.Examples of suitable titanium compounds are TiBr₃, TiBr₄, TiCl₃, TiCl₄,Ti(OCH₃)Cl₃, Ti(OC₂H₅)Cl₃, Ti(O-i-C₃H₇)Cl₃, Ti(O-n-C₄H₉)C₁₃,Ti(OC₂H₅)Br₃, Ti(O-n-C₄H₉)Br₃, Ti(OCH₃)₂Cl₂, Ti(OC₂H₅)C₂C₁₂,Ti(O-n-C₄H₉)₂C₁₂, Ti(OC₂H₅)₂Br₂, Ti(OCH₃)₃Cl, Ti(OC₂H₅)₃Cl,Ti(O-n-C₄H₉)₃Cl, Ti(OC₂H₅)₃Br, Ti(OCH₃)₄, Ti(OC₂H₅)₄ or Ti(O-n-C₄H₉)₄.Preference is given to using titanium compounds which comprise chlorineas the halogen. Preference is likewise given to titanium halides whichcomprise only halogen in addition to titanium and among these especiallytitanium chlorides and in particular titanium tetrachloride. Among thevanadium compounds, particular mention may be made of the vanadiumhalides, the vanadium oxyhalides, the vanadium alkoxides and thevanadium acetylacetonates. Preference is given to vanadium compounds inthe oxidation states 3 to 5.

In the production of the solid component, at least one compound ofmagnesium is preferably additionally used. Suitable compounds of thistype are halogen-comprising magnesium compounds such as magnesiumhalides and in particular the chlorides or bromides and magnesiumcompounds from which the magnesium halides can be obtained in acustomary way, e.g. by reaction with halogenating agents. For thepresent purposes, halogens are chlorine, bromine, iodine or fluorine ormixtures of two or more halogens, with preference being given tochlorine or bromine and in particular chlorine.

Possible halogen-comprising magnesium compounds are in particularmagnesium chlorides or magnesium bromides. Magnesium compounds fromwhich the halides can be obtained are, for example, magnesium alkyls,magnesium aryls, magnesium alkoxy compounds or magnesium aryloxycompounds or Grignard compounds. Suitable halogenating agents are, forexample, halogens, hydrogen halides, SiCl₄ or CCl₄ and preferablychlorine or hydrogen chloride.

Examples of suitable, halogen-free compounds of magnesium arediethylmagnesium, di-n-propylmagnesium, diisopropylmagnesium,di-n-butylmagnesium, di-sec-butylmagnesium, di-tert-butylmagnesium,diamylmagnesium, n-butylethylmagnesium, n-butyl-sec-butylmagnesium,n-butyloctylmagnesium, diphenylmagnesium, diethoxymagnesium,di-n-propyloxymagnesium, diisopropyloxymagnesium,di-n-butyloxymagnesium, di-sec-butyloxymagnesium,di-tert-butyloxymagnesium, diamyloxymagnesium,n-butyloxyethoxymagnesium, n-butyloxy-sec-butyloxymagnesium,n-butyloxyoctyloxymagnesium and diphenoxymagnesium. Among these,preference is given to using n-butylethylmagnesium orn-butyloctylmagnesium.

Examples of Grignard compounds are methylmagnesium chloride,ethylmagnesium chloride, ethylmagnesium bromide, ethylmagnesium iodide,n-propylmagnesium chloride, n-propylmagnesium bromide, n-butylmagnesiumchloride, n-butylmagnesium bromide, sec-butylmagnesium chloride,sec-butylmagnesium bromide, tert-butylmagnesium chloride,tert-butylmagnesium bromide, hexylmagnesium chloride, octylmagnesiumchloride, amylmagnesium chloride, isoamylmagnesium chloride,phenylmagnesium chloride and phenylmagnesium bromide.

As magnesium compounds for producing the particulate solids, preferenceis given to using, apart from magnesium dichloride or magnesiumdibromide, the di(C₁-C₁₀-alkyl)magnesium compounds. Preferably, theZiegler-Natta catalyst comprises a transition metal selected fromtitanium, zirconium, vanadium, chromium.

Catalysts of the Ziegler type are usually polymerized in the presence ofa cocatalyst. Preferred cocatalysts are organometallic compounds ofmetals of groups 1, 2, 12, 13 or 14 of the Periodic Table of Elements,in particular organometallic compounds of metals of group 13 andespecially organoaluminum compounds. Preferred cocatalysts are forexample organometallic alkyls, organometallic alkoxides, ororganometallic halides.

Preferred organometallic compounds comprise lithium alkyls, magnesium orzinc alkyls, magnesium alkyl halides, aluminum alkyls, silicon alkyls,silicon alkoxides and silicon alkyl halides. More preferably, theorganometallic compounds comprise aluminum alkyls and magnesium alkyls.Still more preferably, the organometallic compounds comprise aluminumalkyls, preferably trialkylaluminum compounds. Preferably, the aluminumalkyls comprise, for example, trimethylaluminum, triethylaluminum,tri-isobutylaluminum, tri-n-hexylaluminum and the like.

The polyolefin grows in the form of polymer particles having a more orless regular morphology and size, depending on the catalyst morphologyand size, and on polymerization conditions. Depending on the catalystused, the polyolefin particles usually have a mean diameter of from afew hundred to a few thousand micrometers. In the case of chromiumcatalysts, the mean particle diameter is usually from about 400 to about1600 μm, and in the case of Ziegler-Natta catalysts the mean particlediameter is usually from about 600 to about 3000 μm.

The fluidized-bed reactor, from which the polyolefin particles aretransferred to the second gas-phase reactor, is characterized in beingequipped with a settling pipe for discharging the polyolefin particles.The settling pipe is positioned in a way that its upper opening isintegrated into the distribution grid. Preferably the settling pipe isarranged substantially vertically where substantially vertically meansthat the angle between the longitudinal direction of the settling pipeand the vertical is not more than 40 degrees and preferably not morethan 10 degrees. In a preferred embodiment of the present invention thelower end of the settling pipe tapers conically to prevent a dead zonewhere polymer particles could get stuck.

Polyolefin particles coming from the fluidized bed fall due to gravityinto the settling pipe and form therein a densified bed of polyolefinparticles. At the lower end of the settling pipe, polyolefin particlesare withdrawn and transferred to the second gas-phase reactor.Consequently, the polyolefin particles within the settling pipe movedownwards from top to bottom of the settling pipe driven by gravity.Preferably, the polyolefin particles move as plug flow from top tobottom of the settling pipe.

At its lower end, the settling pipe is provided with a discharge valvethrough which the polyolefins particles are withdrawn from the settlingpipe. Suitable discharge valves are, for example, segmental ball or ballvalves or rotary plug valves. Preferably the discharge valve is asegmental ball valve. By regulating this valve, the discharge flow iscontrolled which allows keeping the bed level inside the fluidized-bedreactor constant. The discharge of the polyolefin particles can becarried out continuously or intermittently. Preferably the polyolefinparticles are continuously withdrawn from the settling pipe. In case thedischarge of the polyolefin particles is carried out intermittently, theopening intervals of the valve are selected in a way that the continuouspolymerization in the fluidized-bed reactor is not or at least onlyminimally disturbed. A fluid is fed into the settling pipe in an amountthat an upward stream of the fluid is induced in the bed of polyolefinparticles above the fluid introduction point. Preferably the fluid isintroduced into the lower third of the settling pipe and especially at aposition near the lower end of the settling pipe. It is also possible tofeed the fluid at more than one position into the settling pipe.Preferably the fluid is fed in a way that it is distributed over thewhole cross-section of the settling pipe in a region above the fluidintroduction point. It can be possible to achieved such a distributionwith simple means for adding a fluid; it is however also possible toutilize a gas distributor. The fed fluid can be a gas or can be aliquid, which evaporates under the conditions in the settling pipe, orcan be a mixture of a gas and such a liquid. Preferably the fluid fedinto the settling pipe is a gas. Accordingly, the introduced fluidreplaces the reaction gas of the fluidized-bed reactor and acts asbarrier, which prevents the reaction gas of the first gas-phasepolymerization reactor from being transferred to the secondpolymerization reactor. The introduced fluid is preferably a componentof the reaction gas mixtures of both the first and the second gas-phasepolymerization reactor. It is preferably an inert component andespecially preferred a saturated hydrocarbon such as propane. Preferablythe amount of fed fluid is regulated in a way that an effective upwardstream of the fluid in the bed of polyolefin particles above the fluidintroduction point is induced and reliably sustained. It is howeverfurther preferred that a not too high amount of fluid is introducedsince on the one hand an expansion of the bed in the settling pipe,which could transport polyolefins back into the fluidized-bed reactor,should be avoided and, on the other hand, the higher the amount of addedinert is the higher is either the dilution of the reaction gas with thefluid or the need for purging a part of reaction gas.

The polyolefin particles withdrawn from the lower end of the settlingpipe are transferred to the second gas-phase reactor. This secondgas-phase reactor can be of any type of commonly used gas-phase reactorsfor preparing polyolefins such as horizontally or vertically stirredgas-phase reactors, fluidized-bed reactors or multizone circulatingreactors, in which two polymerization zones are linked to one anotherand the polymer is passed alternately a plurality of times through thesetwo zones. Preferably the second gas-phase reactor is either afluidized-bed gas reactor or a multizone circulating reactor. Preferredmultizone circulating reactors are, for example, described in WO97/04015 and WO 00/02929 and have two interconnected polymerizationzones, a riser, in which the growing polymer particles flow upward underfast fluidization or transport conditions and a downcomer, in which thegrowing polymer particles flow in a densified form under the action ofgravity. The polymer particles leaving the riser enter the downcomerriser and the polymer particles leaving the downcomer are reintroducedinto the riser, thus establishing a circulation of polymer between thetwo polymerization zones. Such multizone circulating reactors also allowpolymerizing in both polymerization zones under different polymerizationconditions.

The transfer of polyolefin particles according to the present inventiontakes places from one gas-phase reactor to another gas-phase reactor ina multistage polymerization of olefins. The multistage polymerization ofolefins may comprise however also further, additional polymerizationstages carried out in additional reactors. These additionalpolymerization reactors can be any kind of low-pressure polymerizationreactors such as gas-phase reactors or suspension reactors. If themultistage polymerization of olefins includes polymerization insuspension, the suspension polymerization is preferably carried outupstream of the gas-phase polymerization. Suitable reactors for carryingout such a suspension polymerization are for example loop reactors orstirred tank reactors. Suitable suspension media are inter alia inerthydrocarbons such as isobutane or mixtures of hydrocarbons or else themonomers themselves. Such additional polymerization stages in suspensionmay also include a pre-polymerization stage. If the multistagepolymerization of olefins comprises additional polymerization stagescarried out in gas-phase the additional gas-phase polymerizationreactors can be any type of gas-phase reactors like horizontally orvertically stirred gas-phase reactors, fluidized-bed reactors ormultizone circulating reactors. Such additional gas-phase polymerizationreactors may be arranged upstream or downstream the first and the secondgas-phase polymerization reactor of the multistage polymerization ofolefins. In a preferred embodiment of such a multistage polymerizationin more than two gas-phase polymerization reactors all gas-phasereactors, which are not the last gas-phase reactor of the series, arefluidized-bed reactor according the present invention comprising a gasdistribution grid and a settling pipe, which is integrated with itsupper opening into the distribution grid.

FIG. 1 shows schematically the set-up of two serially connectedfluidized-bed reactors for carrying out the process of the presentinvention, i.e. both gas-phase reactors for polymerizing olefins arefluidized-bed reactors.

The first gas-phase reactor, fluidized-bed reactor (1), comprises afluidized bed (2) of polyolefin particles, a gas distribution grid (3)and a velocity reduction zone (4). The velocity reduction zone (4) isgenerally of increased diameter compared to the diameter of thefluidized-bed portion of the reactor. The polyolefin bed is kept in afluidization state by an upwardly flow of gas fed through the gasdistribution grid (3) placed at the bottom portion of the reactor (1).The gaseous stream of the reaction gas leaving the top of the velocityreduction zone (4) via recycle line (5) is compressed by compressor (6),transferred to a heat exchanger (7), in which it is cooled, and thenrecycled to the bottom of the fluidized-bed reactor (1) at a point belowthe gas distribution grid (3) at position (8). The recycle gas can, ifappropriate, be cooled to below the dew point of one or more of therecycle gas components in the heat exchanger so as to operate thereactor with condensed material, i.e. in the condensing mode. Therecycle gas can comprise, besides unreacted monomers, also inertcondensable gases, such as alkanes, as well as inert non-condensablegases, such as nitrogen. Make-up monomers, molecular weight regulators,and optional inert gases can be fed into the reactor (1) at variouspositions, for example via line (9) upstream of the compressor (6); thisnon-limiting the scope of the invention. Generally, the catalyst is fedinto the reactor (1) via a line (10) that is preferably placed in thelower part of the fluidized bed (2).

The fluidized-bed reactor (1) further comprises a settling pipe (11),which is integrated with its upper opening into the gas distributiongrid (3) and is preferably arranged substantially vertical. The settlingpipe (11) may be made of a uniform diameter, or preferably comprisesmore sections having decreasing diameters in the downward direction. Thegas distribution grid (3) may be flat, but preferably is endowed with acone shape in such a way that its downward inclination towards thesettling pipe (11) fosters the entry of the polyolefin particle into thesettling pipe (11) due to gravity. The upper opening of the settlingpipe (11) is preferably located in a central position with respect tothe gas distribution grid (3).

During operation of fluidized-bed reactor (1) the settling pipe (11)contains a bed of polyolefin particles which moves from top to bottom ofthe settling pipe. The polyolefin particles enter the settling pipe (11)through the upper opening and they are withdrawn preferably continuouslythrough discharge valve (12), which is preferably a segmental ballvalve.

A fluid is fed via line (13) into the settling pipe (11), preferably ata position near the lower end of the settling pipe in an amount that anupward stream of the fluid is induced in the bed of polyolefinparticles. The introduced fluid is preferably an inert component andespecially preferred a saturated hydrocarbon such as propane. Thepropane is preferably taken from a gas recovery unit (not shown) inwhich purified propane is obtained by distillation or separation fromoff-gas of the polymerization reactors.

The second gas-phase reactor, fluidized-bed reactor (21) is operatedlike the fluidized-bed reactor (1). It comprises a fluidized bed (22) ofpolyolefin particles, a gas distribution grid (23) and a velocityreduction zone (24). The polyolefin bed is kept in a fluidization stateby an upwardly flow of gas fed through the gas distribution grid (23).The gaseous stream of the reaction gas leaving the top of the velocityreduction zone (24) via recycle line (25) is compressed by compressor(26), transferred to a heat exchanger (27), in which it is cooled, andthen recycled to the bottom of the fluidized-bed reactor (21) at a pointbelow the gas distribution grid (23) at position (28). Make-up monomers,molecular weight regulators, and optional inert gases can be fed intothe reactor (21) for example via line (29) upstream of the compressor(26).

Discharge valve (12) is arranged above a line (20) which branches offthe recycle gas line (25) downstream of the heat exchanger (27). Line(20) carries a part of the recycle gas of the second gas-phase reactor(21). The polyolefin particles having passed discharge valve (12) enterthe line (20) and are transported to the second gas-phase reactor (21)which they enter at point (30). In case the second fluidized-bed reactor(21) is operated in condensing mode it might be advisable to branchesoff line (20) from the recycle gas line (25) upstream of the heatexchanger (27) to avoid that liquid enters line (20).

FIG. 2 shows schematically a set-up of two serially connected gas-phasereactors for carrying out the process of the present invention in whichthe first gas-phase reactor is a fluidized-bed reactor and the secondgas-phase reactor is a circulating gas-phase reactor with twointerconnected reaction zones as described in WO 97/04015 A1 or WO00/02929 A1.

The first gas-phase reactor is a fluidized-bed reactor (1) identical tothe one shown in FIG. 1 and described above.

The second gas-phase reactor is a circulating gas-phase reactor (31)with two reaction zones, riser (32) and downcomer (33), which arerepeatedly passed by the polyolefin particles. Within riser (32), thepolyolefin particles flow upward under fast fluidization conditionsalong the direction of arrow (34). Within downcomer (33) the polyolefinparticles flow downward under the action of gravity along the directionof arrow (35).

The riser (32) and the downcomer (33) are appropriately interconnectedby the interconnection bends (36) and (37). After flowing through theriser (32), the polyolefin particles and the gaseous mixture leave theriser (32) and are conveyed to a solid/gas separation zone (38). Thissolid/gas separation can be effected by using conventional separationmeans such as, for example, a centrifugal separator like a cyclone. Fromthe separation zone (38) the polyolefin particles enter the downcomer(33).

The gaseous mixture leaving the separation zone (38) is recycled to theriser (32) by means of a recycle line (39), equipped with a compressor(40) and a heat exchanger (41). Downstream the heat exchanger (41) therecycle line (39) splits and the gaseous mixture is divided into twoseparated streams: line (42) conveys a part of the recycle gas into theinterconnection bend (37), while the line (43) conveys another part therecycle gas to the bottom of the riser (32), so as to establish fastfluidization conditions therein.

Make-up monomers, make-up monomers, and optionally inert gases can befed to the circulating gas-phase reactor (31) through one or more lines(44), suitably placed at any point of the gas recycle line (39),according to the knowledge of the skilled person in art. The obtainedpolyolefin particles are continuously discharged from the bottom part ofdowncomer (33) via a discharge line (45).

Discharge valve (12) is arranged above a line (46) which branches offthe recycle gas line (39) downstream of heat exchanger (41) or branchesof lines (42) or (43). Line (46) carries a further part of the recyclegas of circulating gas-phase reactor (31). The polyolefin particleshaving passed discharge valve (12) enter the line (46) and aretransported to the circulating gas-phase reactor (31) which they enterat point (47).

The present invention further refers to a process for polymerizingolefins at temperatures of from 30° C. to 140° C. and pressures of from1.0 MPa to 10 MPa in the presence of a polymerization catalyst in amultistage polymerization of olefins in at least two serially connectedgas-phase polymerization reactors, wherein the process for transferringpolyolefin particles from a first gas-phase polymerization reactor to asecond gas-phase polymerization reactor is carried out as describedabove.

Another aspect of the present invention is a reactor for polymerizingolefins in a fluidized bed of polyolefin particles comprising a gasdistribution grid, a velocity reduction zone and recycle gas line fortransferring reaction gas from the top of the velocity reduction zone toa part of the reactor below the gas distribution grid, the recycle gasline being equipped with a compressor and a heat exchanger, wherein thereactor further comprises a settling pipe which is integrated with itsupper opening into the distribution grid. The settling pipe is equipped,at the lower third of the settling pipe, preferably at a position nearthe lower end of the settling pipe with an inlet for introducing a fluidinto the settling pipe and it is further equipped, at its lower end,with an outlet for withdrawing polyolefin particles, preferably shut offby a discharge valve. The settling pipe is preferably arrangedsubstantially vertically. It is further preferred that the lower end ofthe settling pipe tapers conically. Such a reactor is especiallysuitable as first gas-phase reactor in an apparatus for polymerizingolefins comprising two serially connected gas-phase polymerizationreactors for polymerizing olefins in a multistage polymerization. Thepresent invention accordingly also refers to a process for dischargingpolyolefin particles from such a fluidized-bed reactor, wherein a fluidis introduced into the settling pipe in an amount that an upward streamof the fluid is induced in the bed of polyolefin particles above thefluid introduction point and polyolefin particles are withdrawn from thelower end of the settling pipe. Such a discharging process is not onlyadvantageous in combination with transferring the discharged polyolefinparticles to a next gas-phase reactor of a series of polymerizationreactors but can also offer advantages if the discharged polyolefinparticles are fed to degassing and extruding facilities. For example, ifthe polymerization of olefins is a copolymerization of ethylene it ispossible by a discharging process according to the present invention todecrease drastically the amount of comonomer transferred to thedegassing unit. This allows carrying out the polymerization process witha smaller degassing unit which can operate more economically because itrequires less construction costs and has lower operating costs.

EXAMPLES

The melt flow rate MFR_(2.16) was determined according to DIN EN ISO1133:2005, condition D at a temperature of 190° C. under a load of 2.16kg.

The density was determined according to DIN EN ISO 1183-1:2004, Method A(Immersion) with compression molded plaques of 2 mm thickness. Thecompression molded plaques were prepared with a defined thermal history:Pressed at 180° C., 20 MPa for 8 min with subsequent crystallization inboiling water for 30 min.

The particle size distribution was determined through the use of a TylerTesting Sieve Shaker RX-29 Model B available from Combustion EngineeringEndecott provided with a set of six sieves, according to ASTM E-11-87,of number 5, 7, 10, 18, 35, and 200 respectively.

The bulk density was determined according to DIN EN ISO 60:2000-01.

The hydrogen concentration in the second fluidized-bed reactor (21) wasdetermined by gas chromatography.

A homopolymerization of ethylene was carried out in the presence ofhydrogen as molecular weight regulator and propane as inert diluent inthe first fluidized-bed reactor (1) a series of two connectedfluidized-bed reactors as shown in FIG. 1. The cylindrical reaction partof the fluidized bed reactor (1) had an inner diameter of 1000 mm and aheight of 3500 mm. The fluidized-bed reactor (1) was equipped with avertically arranged settling pipe (11), which was integrated with itsupper opening in the gas distribution grid (3). The settling pipe (11)had a cylindrical part with an inner diameter of 200 mm and a length1250 mm and was then conically tapering over a length of 300 mm to theinner diameter of the discharge line of 40 mm. Propane was fed as fluidinto the settling pipe (11) at a position near the lower end of thesettling pipe in order to prevent the gas composition of the firstfluidized-bed reactor from being carried over to the secondfluidized-bed reactors. The utilized discharge valve was intermittentlyopening with an opening time of each times 1 s. The upper level of thefluidized bed was adjusted in a way that the mean residence time of thepolyolefin particles in the first fluidized-bed reactor was always 2.0h.

The second fluidized-bed reactor was not operated as polymerizationreactor but only as take-up device for the transferred polyethyleneparticles and accordingly the gas-phase of the second fluidized-bedreactor was pure propane. To keep the level of the fluidized bed in thesecond reactor constant, the same amount of polymer was discharged fromthe second reactor as was transferred from the first reactor. Thepressure in the second reactor was kept constant by feeding freshpropane to compensate for gas losses in connection with dischargingpolymer particles from the second reactor.

For carrying out the polymerization, a Ziegler catalyst was used whichwas prepared as described in Examples 1-6 of WO 2009/027266. Thepre-polymerized solid catalyst component was then contacted withtriisobutylaluminum (TIBAL) in liquid propane at 40° C. and a pressureof 2.5 MPa in a pre-contacting vessel in a weight ratio of 2 g TIBAL/gcatalyst. The mean residence time of the catalyst in the pre-contactingvessel was 36 min.

Example 1

An ethylene polymerization with a production rate of 80 kg/h was carriedout in the fluidized-bed reactor (1) at 80° C. and a pressure of 2.5MPa. The composition of the reaction gas was 6.5 mol % ethylene, 19.5mol % hydrogen and 74 mol % propane. The produced polyethylene had amelt flow rate MFR_(2.16) of 140 g/10 min, a density of 0.968 g/cm³. Theaverage particle diameter of the obtained polyethylene particles was 970μm, 0.3% of the polyethylene particles had a particle diameter of lessthan 180 μm and the bulk density of the obtained polyethylene particleswas 0.526 g/cm³.

The discharge of the polyethylene particles was carried outintermittently with 50 openings of the discharge valve per hour, thusdischarging in average 1.6 kg of polyethylene particles per opening. Thepolyethylene particles were discharged into the second fluidized-bedreactor which had a pressure of 2.1 MPa.

Propane was fed into the settling pipe in a quantity of 18 kg/h. Aftertwo hours of operating the hydrogen concentration in the secondfluidized-bed reactor (21) remained below the detection limit of 0.1 vol%. This proves that the operating conditions were adequate forpreventing the reaction gas of the first fluidized-bed reactor (1) frombeing transferred into the second fluidized-bed reactor (21).

Example 2

An ethylene polymerization similar to the polymerization of Example 1was carried out; however the production rate was increased to 350 kg/h.

The discharge of the polyethylene particles was carried outintermittently with 220 openings of the discharge valve per hour, thusdischarging in average 1.6 kg of polyethylene particles per opening.Propane was fed into the settling pipe in a quantity of 65 kg/h.

After two hours of operating the hydrogen concentration in the secondfluidized-bed reactor (21) remained below the detection limit of 0.1 vol%, proving that no reaction gas of the first fluidized-bed reactor (1)was transferred into the second fluidized-bed reactor (21).

Example 3

The ethylene polymerization of Example 2 was repeated.

The discharge of the polyethylene particles was carried outintermittently with 220 openings of the discharge valve per hour, thusdischarging in average 1.6 kg of polyethylene particles per opening.Propane was fed into the settling pipe in a quantity of 50 kg/h.

After two hours of operating the hydrogen concentration in the secondfluidized-bed reactor (21) remained below the detection limit of 0.1 vol%, proving that no reaction gas of the first fluidized-bed reactor (1)was transferred into the second fluidized-bed reactor (21).

Comparative Example A

The ethylene polymerization of Example 2 was repeated.

The discharge of the polyethylene particles was carried outintermittently with 120 openings of the discharge valve per hour, thusdischarging in average 1.6 kg of polyethylene particles per opening.Propane was fed into the settling pipe in a quantity of 40 kg/h.

Shortly after starting transferring polyethylene particles from thefirst to the second fluidized-bed reactor, hydrogen could be detected inthe second reactor. After one hour of operating, a hydrogenconcentration of 0.5 vol % was reached showing that reaction gas of thefirst fluidized-bed reactor (1) was transferred into the secondfluidized-bed reactor (21). Accordingly a too low amount of propane wasfed into the settling pipe (11) to achieve an upward stream of propanein the bed of polyethylene particles in the settling pipe (11).

What is claimed is:
 1. A process for transferring polyolefin particlesfrom a first gas-phase polymerization reactor to a second gas-phasepolymerization reactor in a multistage polymerization of olefins carriedout in at least two serially connected gas-phase polymerizationreactors, wherein the first gas-phase reactor is a fluidized-bed reactorcomprising a gas distribution grid and a settling pipe, which isintegrated with its upper opening into the distribution grid andcontains a bed of polyolefin particles which moves from top to bottom ofthe settling pipe, the process comprising the steps of introducing afluid into the settling pipe in an amount that an upward stream of thefluid is induced in the bed of polyolefin particles above the fluidintroduction point; withdrawing polyolefin particles from the lower endof the settling pipe; and transferring the withdrawn polyolefinparticles into the second gas-phase polymerization reactor.
 2. Theprocess for transferring polyolefin particles according to claim 1,wherein the settling pipe is arranged substantially vertically.
 3. Theprocess for transferring polyolefin particles according to claim 1,wherein the lower end of the settling pipe tapers conically.
 4. Theprocess for transferring polyolefin particles according to claim 1,wherein the bed of polyolefin particles moves as plug flow from top tobottom of the settling pipe.
 5. The process for transferring polyolefinparticles according to claim 1, wherein the polyolefin particles arecontinuously withdrawn from the settling pipe.
 6. The process fortransferring polyolefin particles according to claim 1, wherein thepolyolefin particles withdrawn from the settling pipe are transferredinto the second gas-phase polymerization reactor by means of reactiongas of the second gas-phase polymerization reactor.
 7. The process fortransferring polyolefin particles according to claim 1, wherein themultistage polymerization of olefins is a multistage polymerization ofethylene or ethylene and comonomer and the polymerization in the firstpolymerization reactor is a polymerization carried out in the presenceof hydrogen.
 8. The process for transferring polyolefin particlesaccording to claim 1 wherein the fluid introduced into the settling pipeis an inert gas.
 9. The process of claim 1, further comprising the stepof polymerizing olefins at temperatures of from 30° C. to 140° C. andpressures of from 1.0 MPa to 10 MPa in the presence of a polymerizationcatalyst in the multistage polymerization of olefins in at least twoserially connected gas-phase polymerization reactors.